Spark Plasma Sintering Ceramics Research Articles

Fracture resistance of binderless tungsten carbide consolidated by spark plasma sintering and flash sintering

Binderless tungsten carbide (WC) consolidated by spark plasma sintering (SPS) and electrical resistance flash sintering (ERFS) has emerged as a promising alternative to materials obtained by traditional sintering methods. In this study, we investigated the influence of SPS and ERFS techniques on the mechanical properties of binderless WC. Hardness, indentation fracture resistance and elastic modulus were compared and analysed with respect to the microstructure of the resulting material. The results show that SPS tungsten carbide is harder, stiffer and denser when compared to the material produced by ERFS. Nevertheless, the fracture resistance of SPS ceramics was limited due to the lack of macroscopic toughening mechanisms. Conversely, flash-sintered tungsten carbide, consolidated by ERFS, possesses unique biphasic WC/W2C microstructures that promote crack deflection and bridging toughening mechanisms. Additionally, flash-sintered materials exhibit a more ductile character and are characterised by a lower effect of the strain gradient on the material plasticity upon indentation. This study provides valuable insights into the mechanical properties of ultra-hard monolithic ceramics, such as WC, influenced by ultrafast/flash sintering techniques.

Does flash sintering alter the deformation mechanisms of tungsten carbide?

The study aims at unravelling the effect of fast and ultrafast electric current-assisted sintering on the plasticity of ultrahard binderless tungsten carbide. The small-scale deformation of polycrystalline micropillars obtained from samples consolidated by Spark Plasma Sintering (SPS) and Electrical Resistance Flash Sintering (ERFS) processes was studied. Micropillars, 3 µm in diameter, were prepared by focused ion beam and compressed ex and in-situ at room and high temperatures (700 °C). Electron-transparent lamellas were milled out from the pillars plastically deformed at different strain levels to carry out Transmission Kikuchi diffraction (TKD) and HRTEM analyses. At room temperature, the micropillars show similar mechanical responses under compression, reaching outstanding yield strengths (8–11 GPa) with plastic strain not exceeding 3–5% because of a dislocation-assisted toughening mechanism. At 700 °C, the pillar's yield strength drops to around 1.5–2 GPa accompanied by the relevant temperature-activated plasticity. However, only the pillars prepared from flash-sintered material can be homogeneously deformed up to ≈50%; conversely, those derived from SPS ceramics fail macroscopically at strains of ≈20–25% strain upon the localisation of plastic strain at shear bands.

Pressure-assisted sintering and characterization of Nd:YAG ceramic lasers

Spark plasma sintering (SPS) is an advanced one-stage, rapid, near-net shape densification technique combining uniaxial pressure with resistive heating. Various transparent ceramics have been successfully fabricated by SPS, despite the existence of inherent carbon contamination and residual pores. Due to the disk-shape of SPS-processed samples, the technique may be suited for producing thin-disk ceramic laser materials. Nevertheless, an in-depth study of these materials has never been reported. With that goal in mind, the major focus of this study was to characterize the laser performance of Nd:YAG ceramics fabricated by one-stage SPS under conventional (60 MPa) and high (300 MPa) applied pressures. In addition to measuring the lasing slope efficiency and threshold, the passive losses associated with each sample were also evaluated. Surprisingly, it was found that in-line transmittance spectra do not provide accurate predictions of laser performance due to the nature of residual porosity. Moreover, homogeneity and beam quality were assessed, and comparisons were drawn between conventional and high-pressure SPS ceramics. This study lays the groundwork for the future of laser materials fabricated by SPS or similar pressure-assisted techniques.

Influence of the Spark Plasma Sintering temperature on the structure and dielectric properties of BaTi(1-x)ZrxO3 ceramics

In this work, structural and dielectric properties of BaTi(1-x)ZrxO3 (BTZ) ceramics prepared by Spark Plasma Sintering (SPS) from powders obtained via Self-propagating High-temperature Synthesis (SHS) are shown to be strongly affected by the sintering temperature. In addition, a post-annealing treatment in air of the as-prepared ceramics leads to a transition from the hexagonal to the tetragonal and cubic phases. The SPS ceramics corresponding to compositions 0.05 ≤ x ≤ 0.20 and obtained at a sintering temperature of 1200 °C exhibit a standard ferroelectric behavior. In contrast, a diffuse phase transition is observed for the case of ceramics sintered at higher temperatures. Finally, the BTZ ceramic containing 5 at.% of Zr displays the best dielectric permittivity and piezoelectric properties as compared to the other compositions taken into account.

Influence of sintering temperature on structural and optical properties of Y2O3–MgO composite SPS ceramics

The paper studies infrared-transparent 50:50 vol% Y2O3–MgO fine-grained composite ceramics produced via glycine-nitrate process and spark plasma sintering (SPS). Effect of SPS temperature (T = 1100, 1200, 1250 and 1300 °С) on consolidation processes of Y2O3–MgO nanopowders and on ceramics properties has been investigated. Morphology, structural-phase state, density, Vickers microhardness and infrared transmittance of Y2O3–MgO nanocomposites have been studied. Transmittance has been found to increase with the SPS temperature reaching 71%@6000 nm wavelength at T = 1300 °C. Maximal Vickers hardness of 10.9 GPa has been achieved at T = 1200 °C. The correlation between processing conditions of ceramics and indicated characteristics was investigated.

Structural and dielectric properties of giant dielectric Na1/2Sm1/2Cu3Ti4O12 ceramics prepared by reactive sintering methods

Na0.5Sm0.5Cu3Ti4O12 (NSCTO) ceramics have been prepared by reactive sintering of amorphous powder. Spark plasma sintering (SPS) for 10 min at 1025 °C and conventional sintering (CS) for 10 h at 1090 °C have been employed. X-ray diffraction measurements confirmed the pure CCTO-like phase for SPS and CS NSCTO ceramics. The SPS ceramic showed an average grain size of 500 nm, which is much smaller than that of the CS () sample. The impedance spectroscopy measurements revealed an electrically inhomogeneous structure in the prepared ceramics. While the resistivities of grains of both ceramic samples were in the same order of magnitude, the resistivity of grain-boundaries of the CS ceramic was three orders of magnitude greater than that of the SPS ceramic. Both of the samples showed giant dielectric constant () over wide ranges of temperatures and frequencies. Nevertheless, the room-temperature dielectric loss of the SPS NSCTO (3.2 at 1.1 kHz) ceramic sample was higher than that of the CS NSCTO (0.08 at 1.1 kHz) ceramic sample due to the reduced grain-boundary resistivity of the former. Two dielectric relaxations were detected for each sample and attributed to the relaxations in grains and grain-boundaries. The dielectric behavior of the SPS and CS NSCTO ceramics could be interpreted in terms of the internal barrier layer capacitor (IBLC) model.

Comparative study of magnetoelectric BaTiO3–Co0.8Zn0.2Fe2O4 bi-tunable ceramics sintered by Spark Plasma Sintering and classical method

Di-phase 0.66BaTiO3–0.33Co0.8Zn0.2Fe2O4 ceramic composites with randomly mixed phases have been obtained by conventional sintering method (CM) and Spark Plasma Sintering (SPS). The impact of sintering method on the structural, microstructural and functional properties is comparatively analysed. The dielectric properties are dominated by the BaTiO3 phase, with contributions from the interfaces mostly on the dielectric relaxation properties, while the ferrimagnetic character of the composite is derived from the ferrite phase as a sum property. Both CM and SPS ceramics show bi-tunable character, i.e. their permittivity can be modified by the application of both electric and magnetic fields. A high dielectric nonlinearity is mostly characteristic to the ceramics with larger grains. The electrical tunability tends to reduce by the application of a magnetic field an almost is cancelled in the SPS ceramic. The two types of ceramics show opposite signs of their magnetocapacitance properties, indicating that they have different origins. A microwave filter realised with these two types of ceramics have been tested. With increasing magnetic field from 0 to 1.9 kOe (above the magnetic coercive field), the resonance frequency can be up/down shifted, as result of the positive/negative magnetoelectric coupling.

Colossal dielectric behavior of Co-doped TiO2 ceramics: A comparative study

The (Nb + In) co-doped rutile TiO2 ceramics were synthesized by spark plasma sintering (SPS) and traditional solid-state sintering (TS) techniques, respectively. Highly-density co-doped rutile TiO2 ceramics are prepared with the SPS method by 20 min, whereas it takes nearly 30 h with the TS method. It is observed that SPS ceramics showed smaller and uniform grain size, and better densification than TS counterparts. Both sets of the co-doped TiO2 ceramics showed single rutile phase according to X-ray diffraction (XRD) results. X-ray photoelectron spectroscopy (XPS) is adopted to detect and compare the element-valence-state of both sets of the co-doped TiO2 ceramics. The dielectric response, impedance spectroscopy (IS), and the effects of annealing treatment are systematically investigated, which could not only be helpful for optimizing their dielectric properties, but also shed the light on the colossal dielectric mechanism.

Transparent tetragonal-cubic zirconia composite ceramics densified by spark plasma sintering and hot isostatic pressing

Targeting higher toughness transparent ceramics, tetragonal (3 mol % yttria) and cubic (8 mol % yttria) ZrO2 starting powder mixtures were densified by spark plasma sintering (SPS) in vacuum at 1100 °C and post hot isostatic pressing (HIP) in argon at 1100 °C. The influence of the ultra-fine microstructure and phase composition on the fracture resistance and light transmission in the visible and infra-red range was assessed. Of special interest was the influence of a thermal annealing step in air on the transparency of the SPS and SPS-HIP ceramics.

Fast re-oxidation kinetics and conduction pathway in Spark Plasma Sintered ferroelectric ceramics

The re-oxidation kinetics of BaTiO3 ceramics sintered by Spark Plasma Sintering (SPS) was investigated using in-situ impedance spectroscopy. Thanks to the flexibility of the SPS process, the grain size of the dense ceramics was tuned from 0.5μm to 10μm. The re-oxidation kinetics are found to be very fast regardless of the grain size and a full re-oxidation of the ceramics are achieved after 20h of exposure to an ambient environment at only 600°C. The residual density of charge carriers is reduced when using finer starting powders. SPS ceramics made with micrometer size grains demonstrate a residual charge-carrier density that is one tenth that of ceramics made from 10μm particles. Grain-boundary conduction is dominant through fine-grain SPS ceramics. This latter feature is similar to BaTiO3 sintered using the conventional route with 10μm size grain. Finally, the critical grain size for optimal dielectric permittivity is found to shift from 0.7μm in standard ceramics to 1.5μm in SPS ceramics.

Preparation of phase pure, dense fine grained ceramics by conventional and spark plasma sintering of La-substituted BiFeO3 nanoparticles

High density ceramics of the system Bi1−xLaxFeO3, 0≤x≤0.15, have been prepared starting from nanoparticles obtained by mechanosynthesis. The ceramics have been sintered conventionally at 850°C and by spark plasma sintering (SPS). Sintering conditions have been optimized to obtain single phase ceramics, and the microstructure of the ceramics has been compared. Ceramics prepared conventionally present grain sizes from 5μm to less than 1μm, whereas grain sizes by SPS are in the range from 50 to 100nm, which demonstrates that it is possible to obtain nanostructured ceramics of La-substituted BiFeO3 using mechanosynthesis followed by SPS at low temperature (625–650°C). The as-prepared SPS ceramics show low resistivity, indicating some reduction in the samples. However, after an oxidative anneal in air, ceramics are highly insulating at room temperature and electrically homogeneous. The high quality of the ceramics has also been demonstrated by XRD, EDX, Raman and DSC.

Enhanced lithium ionic conductivity and study of the relaxation and giant dielectric properties of spark plasma sintered Li5La3Nb2O12 nanomaterials

Nanocrystalline Li5La3Nb2O12 (LLN) lithium ion conductor with garnet-like structure is synthesized by mechanical milling and solid state reaction routes. The nanopowder has an average grain size of 26nm. The product nanopowder is sintered by conventional (at 900°C) and spark plasma sintering (SPS) at different temperatures of 800°C, 850°C and 900°C. The conventionally sintered ceramics have coarse grained structure with grain size of 1–2μm, whereas the SPS ceramics have nano-sized grains with grain size as small as 50–100nm at low SPS temperatures. The electrical properties have been studied by impedance spectroscopy measurements. The SPS-850 ceramics exhibit ionic conductivity value of 3.7×10−5S/cm at 27°C, which is one order of magnitude larger than the conventionally sintered sample. For the first time we explore two regions of the temperature dependent conductivity below and above room temperature with different activation energies, most probably due to two Li+ ions migration pathways. The relaxation properties of the studied materials have been analyzed in the conductivity and electric modulus formalisms. Different factors are suggested to contribute to the enhanced conductivity including the hopping frequency and the concentration of mobile Li+ ions.

Dielectric and ferroelectric properties of Ba1−xSrxTiO3 ceramics: effects of grain size and ferroelectric domain

Ba1−xSrxTiO3 (x = 0·3, 0·35) ceramics are fabricated by spark plasma sintering (SPS) technique and conventional sintering (CS) route. Their microstructures are determined together with the dielectric and ferroelectric properties. The grain sizes of SPS samples are smaller than those of CS. The differential scanning calorimetry data show that almost no latent heat associated with phase transition can be detected in the SPS sintered ceramics. Raman spectroscopy analysis suggests that the tetragonal phase increases with increasing the grain size, and the higher content of tetragonal phase leads the superior dielectric properties in CS sintered samples. Transmission electron microscopy analysis indicates that domain structure heavily relies on the grain size of ceramics. The poor ferroelectric properties of SPS ceramics are originated from the incomplete development of the tetragonal structure.

Electrical properties of BiFeO3–BaTiO3 ceramics fabricated by mechanochemical synthesis and spark plasma sintering

0.75BiFeO3–0.25BaTiO3 (75BF–25BT) multiferroic powders were prepared by a mechanochemical synthesis at room temperature and the dense ceramics were sintered by a spark plasma sintering (SPS) at 1023–1073K. Microstructures of the ball milled powders and SPS ceramics were thoroughly investigated through X-ray diffraction and scanning electron microscopy, respectively. A bulk density up to 96% of theoretical density was achieved in the SPS ceramics. Dielectric spectra of the 75BF–25BT ceramics were characterized in a broad range of temperature and frequency. Typical saturated ferroelectric hysteresis loops with remnant polarization of 10μC/cm2 were obtained at room temperature, which makes it a good candidate for potential applications.

Comparison of ZrB 2–SiC ceramics with Yb 2O 3 additive prepared by hot pressing and spark plasma sintering

This work compared phase composition, microstructures and mechanical properties of ZrB 2—20 vol.% SiC—5 vol.%Yb 2O 3 prepared by hot pressing (HP) and spark plasma sintering (SPS) in vacuum. Despite the same raw material composition, the two densification techniques led to the appearance of different secondary phases. The HP ceramics had coarsened microstructure, predominantly intergranular fracture characteristic and high fracture toughness. The SPS ceramics exhibited refined microstructure, transgranular fracture model and high bending strength.

Synthesis and Electrical Properties of KF-Doped BaTiO3 Ceramics

KF-doped barium titanate ceramics, Ba1-xKxTiO3-xFx (KF-BT/x: 0≦x≦0.15), were fabricated by spark plasma sintering (SPS) using fine particles prepared by a sol–gel method. We measured the dielectric and piezoelectric properties of the SPS ceramics after annealing in an O2 gas flow at 1273 K. The dielectric constant and the piezoelectric constant d33 were enhanced in KF-BT/0.10, as found in the single crystals, because the composition of x=0.10 locates near the tricritical point (TCP). However, some different behaviors from those of the single crystals were found; in particular, TC of the ceramic forms is much higher than that of the crystalline forms, and the difference becomes marked with increasing x. We consider that the evaporation of F-, which occurs during the O2 annealing process, markedly increases TC.

Low-losses, highly tunable Ba0.6Sr0.4TiO3∕MgO composite

Spark plasma sintering (SPS) is an efficient tool to obtain highly densified ferroelectric-dielectric ceramic composites with clean interfaces and tunable properties. Dielectric MgO and ferroelectric Ba0.6Sr0.4TiO3 (BST) were combined in two-dimensional multilayer and three-dimensional random powders design. Their unmodified BST Curie temperature proves the suppression of interdiffusion while dielectric losses are below 0.5% and the tunability is 40% at room temperature. The composites and pure BST with similar densities (>95%) were obtained, owing reliable comparison of their dielectric properties. Such SPS ceramics can be used as experimental input for simulation and are potential candidates for high frequency applications.

PMN–PT Ceramics Prepared By Spark Plasma Sintering

(1−x)Pb(Mg1/2Nb2/3)O3−xPbTiO3 (PMN–PT) ceramics of stoichiometric composition were fabricated by conventional pressureless sintering (CS) and spark plasma sintering (SPS). The CS ceramics exhibited a change from relaxor to normal ferroelectric behavior (FE) with increasing PT content. However, low dielectric constants, frequency dispersion, and diffuse phase transition behavior typical for relaxors were obtained for all SPS ceramics. FE and piezoelectric measurements further demonstrated low remanent polarization and strain, high coercive field, and low electromechanical response from SPS materials. Normal dielectric and enhanced FE performance appeared following high‐temperature heat treatment after SPS. The effects of grain size, microstructure, and chemical heterogeneity formed during fast sintering are considered.

Dielectric Properties of Spark Plasma Sintered (SPS) Barium Strontium Titanate (BST) Ceramics

Grain size can have a very important effect on the dielectric properties of ferroelectric ceramics through the interaction of domain structures and grain boundaries. In this work, Ba0.6Sr0.4TiO3 ceramics were fabricated to controlled grain sizes using a spark plasma sintering (SPS) technique which can produce dense ceramics at greatly reduced sintering temperatures and times. The microstructure and dielectric properties of SPS produced ceramics have been characterised and compared with those from conventional presureless sintered ceramics. The results show that SPS ceramics exhibit a duplex microstructure with 1-3 μ m BST grains surrounded by submicron BT and ST grains. Consequently, they show a significantly diffused transition in permittivity and reduced dielectric losses in the ferroelectric region compared to conventionally sintered ceramics. This is believed to result largely from the large differences in grain sizes and domain structures. The mechanisms will be discussed in terms of domain walls contributions.

Electrical property of 8-mol% yttria-stabilized zirco-nia electrolyte by spark-plasma sintering

Spark-plasma sintering (SPS) process was used to sinter ZrO2 (8Y) powders, and a relative density of 99% has been reached at a low temperature (1350°C) and short dwelling time (10 min) compared to the conventional sintered identical samples (1450°C 4 h)(CS). By the X-ray diffraction (XRD) patterns, the microstructure of the SPS pellet and CS pellet are both indexed by the cubic unit cell (Fm3m). The finer crystallite size (D111) of the SPS pellet is 154 nm and D111 of the CS pellet is more than 1 μm. The AC impedance spectroscopy shows that the ionic conductivity of SPS pellet is different from that of CS pellet. The activation energy for the SPS ceramics, estimated from the slope in the range of 400–800°C, was 91 kJ mol−1, which is in good agreement with CS pellet (96 kJ mol−1), indicating that the conduction mechanism in SPS ceramics is similar to that in CS ceramics.